The company is attempting a U-turn. Windows 8 was Microsoft’s biggest bid so far to adjust its flagship product to the new world of touch-screen devices. Out went the “start” button that had controlled access to the computer’s menu since 1995. In came giant multicoloured tiles that respond to the touch. Steve Ballmer, Microsoft’s boss, described the introduction of the new system as a “bet-the-company” moment. But the bet proved so ill-judged that an app which lets users reintroduce the familiar start button is now one of the bestsellers for Windows 8.